DESCRIPTION: The angular and linear vestibulo-ocular reflexes (aVOR and lVOR) can be characterized by compensatory and spatial orientation mechanisms. The compensatory mechanisms move the eyes to counter head movement so as to keep the eyes stable in space. Spatial orientation mechanisms tend to align ocular rotations or the yaw axis of the eye with the gravito-inertial acceleration (GIA). The compensatory IVOR has been modeled by relating its gain and phase characteristics to the behavior of a range of highly regular to highly irregular otolith afferents. An algorithm has been determined to show how the central vestibular system might combine the output of the otolith afferents so as to implement the compensatory function of the lVOR. It is proposed to extend the modeling approach to encompass orientation responses such as ocular counter-rolling and vertical ocular tilts due to rotations of the GIA about the naso-occipital and interaural axes. Homogeneous transformation matrices will be used to model vergence due to naso-occipital acceleration. Effects of vision and viewing distance on the lVOR will be modeled as a function of vergence. Model-based experiments are proposed to identify the parameters of the orientation system that generates ocular counter-rolling and vertical tilts. Using sinusoidal centrifugation, tangential acceleration at low velocities will identify the high frequency characteristics of the lVOR compensatory and orienting mechanisms and centripetal accelerations will be used to identify the low frequency characteristics. Spatio-temporal convergent (STC) otolith related units in the vestibular nuclei will be studied to determine whether they code a linear combination of otolith afferent behavior. It is expected that these neurons will segregate the translation and tilt responses and will have characteristics of the compensatory and orienting subsystems. The model will also be combined with the spatial orientation model of velocity storage to predict the orientation and compensatory behavior during off-vertical axis rotation (OVAR). It is expected that the orienting component of the lVOR will modulate the time constant of velocity storage in three dimensions and cause ocular counter-rolling, vertical tilt, and that the compensatory mechanisms will contribute to the oscillations in horizontal eye velocity. Whether convergence has properties of a compensatory or orienting system will be determined from its amplitude and phase characteristics. When the model of lVOR-aVOR interaction has been established, it will be used to predict the response to pitch while rotating (PWR) for centered and off-axis rotation. This stimulus activates the lVOR, aVOR, and vergence mechanisms in an analogous manner to that which occurs during circular locomotion. These studies should help us understand how the lVOR fosters clear vision during locomotion and help diagnose diseases associated with visual stabilization while walking.